The use of medical imaging has increased dramatically over the last decade, particularly imaging with computed tomography (CT). Integrating CT into routine care has had an extremely positive impact on patient health, and the development of CT is widely considered one of the most important advances in medicine. However, the radiation doses associated with CT are substantial. For example, a chest CT can deliver 50-500 times the radiation dose of a routine chest x-ray. Therefore, along with the increased utilization of CT has come a marked increase in the medical exposure to ionizing radiation. Extensive epidemiological evidence has linked exposure to ionizing radiation with development of radiation-induced cancers, particularly to radiation-sensitive organs, e.g., the thyroid, breast, and hematopoetic systems. The risk to an individual patient is probably relatively small;but many people are exposed annually, with increasingly large exposures per examination from technological advances in imaging and the use of multiple-phase studies. This could translate into many cancer cases resulting directly from the radiation associated with imaging. Additionally, patients who undergo repeated medical imaging, and children who undergo any imaging, may be at particularly high risk. It is important to understand the radiation delivered through medical imaging, so this potential harm of imaging can be weighed against its benefits. Few data are available regarding current exposure to medical radiation in the United States. Understanding exposures to medical radiation is a crucial first step to develop reasonable strategies to minimize unnecessary exposure. The aims of the proposed study are to identify patterns in the utilization of diagnostic medical imaging that involves exposure to ionizing radiation;to quantitatively estimate average annual exposures to ionizing radiation associated with medical imaging and to identify patient and physician factors that are associated with the receipt of particularly high doses of radiation. To pursue these aims, we will conduct a retrospective cohort analysis of patients enrolled in Group Health, a large, mixed-model, nonprofit, healthcare system in Seattle, WA, whose 500,000 members underwent approximately 6.5 million imaging tests between 1993 and 2007. Group Health collects and stores automated data, including detailed clinical records, and the indication and result of all radiological imaging, conducted by physicians of all specialties. Using this large, population-based database, we will describe patterns of diagnostic imaging over time, estimate temporal trends in radiation doses associated with imaging, and determine factors associated with having the highest exposure to medical imaging radiation. Completing these aims will provide quantitative data on the magnitude of the problem of radiation associated with medical imaging. Further, if we validate that Group Health data can be used for such research, future studies will use Group Health data to evaluate the relative risk of cancer associated with ionizing radiation. The ultimate goal of this research is to help keep medical exposure to ionizing radiation to a minimum. PUBLIC HEALTH RELEVANCE: The use of medical imaging has increased dramatically over the last decade, particularly imaging with computed tomography (CT), originally called computerized axial tomography (CAT) scans. Integrating CT into routine care has improved patient health and the development of CT is widely considered one of the most important advances in medicine. However, substantial radiation doses are associated with CT. For example, a chest CT can deliver 50-500 times the radiation dose of a routine chest x-ray. So along with the increased use of CT has come a marked increase in the medical exposure to ionizing radiation. Much epidemiological evidence has linked exposure to ionizing radiation with development of radiation-induced cancers. Little data are available about current exposure to medical radiation in the United States. Understanding exposures to medical radiation is a crucial first step toward developing reasonable strategies to minimize unnecessary exposure. The aims of the proposed study are to identify patterns in the use of diagnostic medical imaging that involves exposure to ionizing radiation;to estimate average annual exposures to ionizing radiation associated with medical imaging;and to identify patient and physician factors associated with receiving particularly high doses of radiation. Subsequent research will quantify the increased relative risk of developing cancer among individuals who have received particularly high doses of radiation from medical imaging. Completing the proposed aims will help measure the problem of radiation associated with medical imaging. The ultimate goal of this research is to help keep medical exposure to ionizing radiation to a minimum. .